A conventional method of adjusting a position of a convergence aperture for defining an irradiation angle in a scanning transmission electron microscope system is carried out by periodically varying a current on an objective lens and mechanically adjusting a position of a convergence aperture in such a manner as to minimize an amount of runoff of a scanning transmission image, or by aligning an optical axis of an electron beam with a position of the aperture using an electromagnetic deflector. In another method, an electron beam is stopped at a certain point on a sample while using a convergence aperture having a large hole diameter or opening a convergence aperture, and a figure appearing on an image passing through the sample is used for adjustment. The image used in the latter method is called a Ronchigram.
As shown in FIG. 2, a flat contrast region 1 located at a central part and an intense contrast region 2 in a polygonal shape located around the flat contrast region 1 are observed together in a Ronchigram obtained by a scanning transmission electron microscope equipped with an aberration corrector. The intense contrast region 2 is derived from an uncorrectable or remaining high-order aberration. The center of an optical axis exists in the flat contrast region 1, and an operator performs adjustment in such a manner as to align the center of the optical axis with the center of the convergence aperture by observing the Ronchigram.
Such a scanning transmission electron microscope equipped with an aberration corrector requires precise positional alignment of a convergence aperture. This is because if the position of the convergence aperture is not accurate, a state of convergence of an electron beam deviates from an aberration-corrected condition and resolution of a scanning transmission image may be deteriorated by introduction of a high-order aberration such as a coma aberration.